<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="d1"><br>
<input type="text" id="d2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)"><br>
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h1>结果:<span></span></h1>
<script>
  let i1=document.querySelector("#d1");
  let i2=document.querySelector("#d2");
  let s=document.querySelector("span")
  function f(x) {
    if (isNaN(i1.value)||isNaN(i2.value)){
      s.innerText="输入错误";
      return;
    }
    switch (x) {
        case 1:
          s.innerText=i1.value*1+i2.value*1;
          break;
      case 2:
        s.innerText=i1.value-i2.value;
        break;
      case 3:
        s.innerText=i1.value*i2.value;
        break;
      case 4:
        s.innerText=i1.value/i2.value;
        break;
    }
  }
</script>
</body>
</html>